can view fixes

CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Grid will now size children to the maximum height of a row https://github.com/Textualize/textual/pull/5113
 - Markdown links will be opened with `App.open_url` automatically https://github.com/Textualize/textual/pull/5113
 - The universal selector (`*`) will now not match widgets with the class `-textual-system` (scrollbars, notifications etc) https://github.com/Textualize/textual/pull/5113
+- Renamed `Screen.can_view` and `Widget.can_view` to `Screen.can_view_entire` and `Widget.can_view_entire` https://github.com/Textualize/textual/pull/5174
 
 ### Added
 
@@ -26,6 +27,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Added `textual._loop.loop_from_index` https://github.com/Textualize/textual/pull/5164
 - Added `min_color` and `max_color` to Sparklines constructor, which take precedence over CSS https://github.com/Textualize/textual/pull/5174
 - Added new demo `python -m textual`, not *quite* finished but better than the old one https://github.com/Textualize/textual/pull/5174
+- Added `Screen.can_view_partial` and `Widget.can_view_partial` https://github.com/Textualize/textual/pull/5174
 
 ### Fixed
 

src/textual/demo/widgets.py

@@ -339,25 +339,25 @@ def on_mount(self) -> None:
         log = self.query_one(Log)
         rich_log = self.query_one(RichLog)
         log.write("I am a Log Widget")
-        rich_log.write("I am a [b]Rich Log Widget")
+        rich_log.write("I am a Rich Log Widget")
         self.set_interval(0.25, self.update_log)
         self.set_interval(1, self.update_rich_log)
 
     def update_log(self) -> None:
         """Update the Log with new content."""
-        if not self.screen.can_view(self) or not self.screen.is_active:
+        log = self.query_one(Log)
+        if not self.screen.can_view_partial(log) or not self.screen.is_active:
             return
         self.log_count += 1
-        log = self.query_one(Log)
         line_no = self.log_count % len(self.TEXT)
         line = self.TEXT[self.log_count % len(self.TEXT)]
         log.write_line(f"fear[{line_no}] = {line!r}")
 
     def update_rich_log(self) -> None:
         """Update the Rich Log with content."""
-        if not self.screen.can_view(self) or not self.screen.is_active:
-            return
         rich_log = self.query_one(RichLog)
+        if not self.screen.can_view_partial(rich_log) or not self.screen.is_active:
+            return
         self.rich_log_count += 1
         log_option = self.rich_log_count % 3
         if log_option == 0:
@@ -425,7 +425,7 @@ def on_mount(self) -> None:
 
     def update_sparks(self) -> None:
         """Update the sparks data."""
-        if not self.screen.can_view(self) or not self.screen.is_active:
+        if not self.screen.can_view_partial(self) or not self.screen.is_active:
             return
         self.count += 1
         offset = self.count * 40

src/textual/screen.py

@@ -896,7 +896,7 @@ def set_focus(self, widget: Widget | None, scroll_visible: bool = True) -> None:
 
                     def scroll_to_center(widget: Widget) -> None:
                         """Scroll to center (after a refresh)."""
-                        if self.focused is widget and not self.can_view(widget):
+                        if self.focused is widget and not self.can_view_entire(widget):
                             self.scroll_to_center(widget, origin_visible=True)
 
                     self.call_later(scroll_to_center, widget)
@@ -1480,24 +1480,44 @@ async def action_dismiss(self, result: ScreenResultType | None = None) -> None:
         await self._flush_next_callbacks()
         self.dismiss(result)
 
-    def can_view(self, widget: Widget) -> bool:
-        """Check if a given widget is in the current view (scrollable area).
+    def can_view_entire(self, widget: Widget) -> bool:
+        """Check if a given widget is fully within the current screen.
 
         Note: This doesn't necessarily equate to a widget being visible.
         There are other reasons why a widget may not be visible.
 
         Args:
-            widget: A widget that is a descendant of self.
+            widget: A widget.
 
         Returns:
-            True if the entire widget is in view, False if it is partially visible or not in view.
+            `True` if the entire widget is in view, `False` if it is partially visible or not in view.
         """
+        if widget not in self._compositor.visible_widgets:
+            return False
+        # If the widget is one that overlays the screen...
+        if widget.styles.overlay == "screen":
+            # ...simply check if it's within the screen's region.
+            return widget.region in self.region
+        # Failing that fall back to normal checking.
+        return super().can_view_entire(widget)
+
+    def can_view_partial(self, widget: Widget) -> bool:
+        """Check if a given widget is at least partially within the current view.
+
+        Args:
+            widget: A widget.
+
+        Returns:
+            `True` if the any part of the widget is in view, `False` if it is completely outside of the screen.
+        """
+        if widget not in self._compositor.visible_widgets:
+            return False
         # If the widget is one that overlays the screen...
         if widget.styles.overlay == "screen":
             # ...simply check if it's within the screen's region.
             return widget.region in self.region
         # Failing that fall back to normal checking.
-        return super().can_view(widget)
+        return super().can_view_partial(widget)
 
     def validate_title(self, title: Any) -> str | None:
         """Ensure the title is a string or `None`."""

src/textual/widget.py

@@ -3303,8 +3303,8 @@ def scroll_to_center(
             immediate=immediate,
         )
 
-    def can_view(self, widget: Widget) -> bool:
-        """Check if a given widget is in the current view (scrollable area).
+    def can_view_entire(self, widget: Widget) -> bool:
+        """Check if a given widget is *fully* within the current view (scrollable area).
 
         Note: This doesn't necessarily equate to a widget being visible.
         There are other reasons why a widget may not be visible.
@@ -3313,11 +3313,38 @@ def can_view(self, widget: Widget) -> bool:
             widget: A widget that is a descendant of self.
 
         Returns:
-            True if the entire widget is in view, False if it is partially visible or not in view.
+            `True` if the entire widget is in view, `False` if it is partially visible or not in view.
         """
         if widget is self:
             return True
 
+        if widget not in self.screen._compositor.visible_widgets:
+            return False
+
+        region = widget.region
+        node: Widget = widget
+
+        while isinstance(node.parent, Widget) and node is not self:
+            if region not in node.parent.scrollable_content_region:
+                return False
+            node = node.parent
+        return True
+
+    def can_view_partial(self, widget: Widget) -> bool:
+        """Check if a given widget at least partially visible within the current view (scrollable area).
+
+        Args:
+            widget: A widget that is a descendant of self.
+
+        Returns:
+            `True` if any part of the widget is visible, `False` if it is outside of the viewable area.
+        """
+        if widget is self:
+            return True
+
+        if widget not in self.screen._compositor.visible_widgets or not widget.display:
+            return False
+
         region = widget.region
         node: Widget = widget